All archaea and bacteria are microbial species living things too small to see with the naked eye and represent a vast number of different evolutionary lineages. Some of these eukaryotic groups contain microbial species, too. Bacteria and archaea may seem pretty similar, but there are some major differences between the two groups.
Archaea can also generate energy differently and have unique ecological roles to play, such as being responsible for producing biological methane—something no eukaryotes or bacteria can do.
These differences may not seem like a big deal to most people—why, then, are they in different groups? By comparing the genomes of different organisms and studying the rate at which genetic changes occur over time, scientists can trace the evolutionary histories of living things and estimate when each group formed a new branch of the tree of life. The molecular and genetic differences between archaea and other living things are profound and ancient enough to warrant an entirely separate domain.
Archaea are famous for their love of living in extreme environments. However, scientists are slowly learning more, helped by new techniques and technologies that make it easier to discover these species in the first place. Methods such as metagenomics allow for the study of genetic material without the need to grow cultures of a particular species in a lab, allowing researchers to study the genetic blueprints of more microbes than ever before. Extreme halophiles are found in places such as the Dead Sea, the Great Salt Lake and Lake Assal which have salt concentrations much higher than ocean water.
Other organisms die in extremely salty conditions. High concentrations of salt draw the water out of cells and cause them to die of dehydration. Extreme halophiles have evolved adaptations to prevent their cells from losing too much water.
Archaea that are found in extremely hot environments are known as extreme thermophiles. Most organisms die in extremely hot conditions because the heat damages the shape and structure of the DNA and proteins found in their cells. Acidophiles are organisms that love highly acidic conditions such as our stomachs and sulfuric pools.
Acidophiles have various methods for protecting themselves from the highly acidic conditions. Structural changes to the cellular membranes can prevent acid entering their cell.
Channels in the membrane of their cell can be used to pump hydrogen ions out of the cell to maintain the pH inside the cell. Methanogens are a group of archaea that produce methane gas as a part of their metabolism.
They are anaerobic microorganisms that use carbon dioxide and hydrogen to produce energy. Methane is produced as a byproduct. Methanogens are anaerobic archaea and are poisoned by oxygen. They are commonly found in the soil of wetlands where all the oxygen has been depleted by other microorganisms. They are also found in the guts of some animals such as sheep and cattle. Methanogens found in the guts of animals help with the digestion of food. Methanogens are also used to treat sewage. Each year, methanogens release around two billion tonnes of methane into the atmosphere.
Methane is a greenhouse gas involved in global warming and climate change. Very little is known about the evolutionary tree of the Domain Archaea. Currently, it is separated into four evolutionary groups which are likely to change as we discover more about these microscopic organisms. The four current clades of archaea are Korarchaeotes, Euryarchaeotes, Crenarchaeotes, and Nanoarchaeotes.
Euryarchaeotes are one of the best-known groups of archaea. It includes a range of extreme halophiles lovers of salt and all methanogens. Some of these extreme halophiles are used in commercial salt production to help speed up the evaporation of saltwater ponds. Some euryarchaeotes have a unique way of using light energy to produce food. Instead of using the well-known pigments, such as chlorophyll a , some euryarchaeotes use a combination of a protein and a pigment called retinal to trap light energy.
Retinal is also a key molecule involved in vision for animals. Thermoproteus has a cellular membrane in which lipids form a monolayer rather than a bilayer, which is typical for archaea. Its metabolism is autotrophic. To synthesize ATP, Thermoproteus spp. The phylum Euryarchaeota includes several distinct classes. Species in the classes Methanobacteria, Methanococci, and Methanomicrobia represent Archaea that can be generally described as methanogens. Methanogens are unique in that they can reduce carbon dioxide in the presence of hydrogen, producing methane.
They can live in the most extreme environments and can reproduce at temperatures varying from below freezing to boiling. Methanogens have been found in hot springs as well as deep under ice in Greenland. Some scientists have even hypothesized that methanogens may inhabit the planet Mars because the mixture of gases produced by methanogens resembles the makeup of the Martian atmosphere.
Some genera of methanogens, notably Methanosarcina , can grow and produce methane in the presence of oxygen, although the vast majority are strict anaerobes. Halobacteria require a very high concentrations of sodium chloride in their aquatic environment. One remarkable feature of these organisms is that they perform photosynthesis using the protein bacteriorhodopsin , which gives them, and the bodies of water they inhabit, a beautiful purple color Figure 2. Figure 2. Halobacteria growing in these salt ponds gives them a distinct purple color.
Notable species of Halobacteria include Halobacterium salinarum , which may be the oldest living organism on earth; scientists have isolated its DNA from fossils that are million years old.
Archaea are not known to cause any disease in humans, animals, plants, bacteria, or in other archaea. Although this makes sense for the extremophiles, not all archaea live in extreme environments. Many genera and species of Archaea are mesophiles, so they can live in human and animal microbiomes, although they rarely do. As we have learned, some methanogens exist in the human gastrointestinal tract.
Yet we have no reliable evidence pointing to any archaean as the causative agent of any human disease. Still, scientists have attempted to find links between human disease and archaea. For example, in , Lepp et al. The authors suggested that the activity of these methanogens causes the disease.
It seems more likely that periodontal disease causes an enlargement of anaerobic regions in the mouth that are subsequently populated by M. There remains no good answer as to why archaea do not seem to be pathogenic, but scientists continue to speculate and hope to find the answer.
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